1. Field of the Invention
The invention relates to a movement detector for a composite image signal, and more particularly to a movement detector resistant to phase errors in the composite signal.
2. Description of the Related Art
For composite PAL (Phase Alternating Line) signal decoding, two adjacent frames of a composite signal are not the same phase when sampling the video signal in phase with one of the U or V axes. Therefore, they can not be subtracted directly to derive the degree of movement. However, when sampling the video signal at 45 degrees off the U or V axes, two adjacent frames of a TV signal can have the same phase and can be subtracted to derive a movement signal.
FIG. 1 is a schematic diagram of an ideal PAL vertical chrominance example. In FIG. 1, f1, f2, f3 and f4 indicates four continuous frames, and the pixels P11 to P14, P21 to P24, P31 to P34, and P41 to P44 are pixels at the same location in different frames. Take the pixels in the dotted frame 11 for example, Ideally, the in-phase pixel of the pixel P11 in frame f1 is the pixel P41 in the frame f4, in other words, the conventional movement signal detection method requires memory for storing at least four frames to derive the movement signal. If the received signal drifts or the received signal is not sampled at 45 degrees, incorrect movement signal detection and the required memory increases may occur.
Please refer to FIG. 2. In FIG. 2, the vectors shown in 21, 22, and 23 indicates three pixels of three continuous lines. In 21, 22 and 23, a phase error θ occurs. Ideally, the received signal has the same phase error, such as shown in FIG. 2, however, the phase error may be not θ in 22 and 23, and this might cause incorrect movement signal detection.
Due to bad composite encoding or noise interference, there may be phase error when sampling the video signal at 45 degrees off the U or V axes. This phase error affects the accuracy of movement detection. This invention discloses a method for reducing the interference of phase error in movement detection.